There are 12 preceding months so the plural "months" should be used. Is this correct? But the chart uses the singular "month" so maybe the singular is correct?
The line graph gives information about price changes of three metals in each month of 2014, compared to the preceding months.
The line graph gives information about price changes of three metals in each month of 2014, compared to the preceding month.
I wrote these two sentences.
The confusion is just over what data is being graphed. The Y axis is labeled "Percent change in price compared with previous month." The value is not a price, it's a change in price. In January, nickel didn't cost "6"; it cost six percent more than it did in December.
As for the proposed sentence, perhaps the best version is simply "The line graph gives information about price changes of three metals in each month of 2014." That's true on its own, and is to-the-point; the "... compared to the preceding month[s]" phrase isn't really necessary.
Note: 1) Yes, it's true that there are many "previous months." But if we say "the previous month," it means the (single) immediately-previous one. And 2) the "the" is left out here because the label uses "headlinese."
Each month is only being compared to one other month, the one immediately preceding it. We use the singular to refer to that one corresponding month.
As another example of one-to-one correspondences like this, suppose a bunch of people are standing in a line. You might say to them
Say "hello" to the person to the right of you.
Even though multiple people will be greeted, each person only greets one person.
On the other hand, you would say
Say "hello" to the people next to you.
if you want each person to greet the person on their right and also the person on their left, since they're each greeting multiple people.
